1. Field of the Invention
The present invention relates generally to the performance of a marine seismic acquisition survey, and, more particularly, to the control of the seismic survey spread during the survey.
2. Background of the Related Art
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. In order to perform a 3-D marine seismic acquisition survey, an array of marine seismic streamers, each typically several thousand meters long and containing a large number of hydrophones and associated electronic equipment distributed along its length, is towed at about 5 knots behind a seismic survey vessel. The vessel also tows one or more seismic sources suitable for use in water, typically air guns. Acoustic signals, or “shots,” produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones carried in the streamers, digitized, and then transmitted to the seismic survey vessel where the digitized signals are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
Often two or more sets of seismic data signals are obtained from the same subsurface area. These sets of seismic data signals may be obtained, for instance, by conducting two or more seismic surveys over the same subsurface area at different times, typically with time lapses between the seismic surveys varying between a few months and a few years. In some cases, the seismic data signals will be acquired to monitor changes in subsurface reservoirs caused by the production of hydrocarbons. The acquisition and processing of time-lapsed three dimensional seismic data signals over a particular subsurface area (commonly referred to in the industry as “4-D” seismic data) has emerged in recent years as an important new seismic prospecting methodology.
It is common practice for a certain amount of information about the survey area to be gathered beforehand so that the appropriate equipment and methods can be selected (known as the “survey design”) to achieve the desired geophysical and operational objectives. Some of this information is used to provide the basic parameters for the survey, such as the boundaries of the survey area, the lengths of the towed streamer cables, and the firing of the seismic sources. Such information has, to some extent, been used to assist in survey control through various independent systems. Typical of such control systems have been vessel autopilots, ship heading control, and towed cable positioning and depth adjustment. For example, U.S. Pat. No. 6,629,037 describes the use of cost maps to optimize paths for seismic in-fill shooting within a known survey area. British Patent Application No. GB 2,364,388 discloses the positioning of seismic sources and streamers within a known survey area according to recorded position data from a prior survey.
It is also well known for a certain amount of information about the survey execution to be gathered during the survey (i.e., in real time or near-real time) so that the appropriate settings and positions can be achieved according to the desired geophysical and operational objectives. Such information has, to some extent, also been used to provide survey control through various independent systems. The state of the art in such control systems is represented by the following patent references: U.S. Pat. No. 6,618,321 (simulation of streamer positioning during a survey according to current determination); U.S. Pat. No. 6,590,831 (coordination of multiple seismic acquisition vessels during a survey according to monitored survey parameters); U.S. Pat. No. 6,418,378 (neural network trained by survey-acquired data for predicting seismic streamer shape during a subsequent survey);U.S. Pat. No. 5,790,472 (positioning of seismic streamers during a survey according to hydrophone noise levels); and International Patent Application No. WO 00/20895 (seismic streamer positioning during a survey according to estimated velocity of streamer positioning devices).
The control systems described above rely upon particular inputs (e.g., marine current) to determine information (e.g., passive streamer shape) useful in controlling a seismic survey towing vessel. None of these systems, however, relies upon or takes into account a broad spectrum of input conditions and parameters that includes the various objectives and constraints of the seismic survey equipment and methods. Furthermore, none of these systems seeks to actively control the spread with a coordinated suite of steering devices deployed throughout the spread. A need therefore exists for such a comprehensive system.
The control systems mentioned above have been designed to achieve desired results by providing outputs, such as commands or paths, for immediate implementation. There has been little or no consideration in such optimization of the important time-delayed effects of these outputs. A need therefore exists for a seismic survey control system that accounts for time-delayed effects of outputs—particularly control commands—as well as the immediate effects.